Recently, small-sized computers such as mini computers and personal computers have been widely employed in business. For this reason, it has become very important to store computer data with high security, and therefore magnetic recording tapes which are named "back-up tapes" have been employed for that purpose. The magnetic recording tapes for computer storage are further required to have great storage capacity so as to store a great amount of computer data in the form of digital data with high security and high reliability.
The back-up tape comprises a flexible plastic web (i.e., support) and a magnetic recording layer provided thereon, in the same manner as in the conventional magnetic recording tapes such as an audio tape and a video tape. The magnetic recording layer comprises a ferromagnetic powder and a binder.
The magnetic recording tape is repeatedly run in contact with a magnetic head. The back-up tape sometimes is more repeatedly run than the conventional audio or video tape.
The back-up tape is encased in a cassette, in the same manner as for an audio tape. The most easy way to increase the data storage capacity of a magnetic recording tape is to increase the length of the tape. However, the size of the compact cassette for encasing the back-up tape cannot be freely enlarged. Therefore, if the length of the back-tape is desired to increase, the thickness of the tape should be decreased. For this reason, the most back-up tape has a decreased thickness, as compared with the conventional audio tape.
The increased data storage capacity of a magnetic recording tape also can be attained by decreasing the size of the ferromagnetic powder and decreasing the thickness of the magnetic recording layer. The back-up tape also should have improved electromagnetic characteristics, and the improvement of electromagnetic characteristics can be accomplished by giving to the magnetic recording layer high smoothness.
The decreased tape thickness and increased smoothness of the surface of the magnetic recording layer sometimes cause tape running troubles such as wrong tape winding. For the reason, a back-coating layer is generally provided on the reverse side of the back-up tape, that is, on the surface of the support opposite to the surface on which the magnetic recording layer is provided. The back-coating layer naturally should be thin so as not to extremely increase the tape thickness.
In the digital data storage systems employed for recording the computer data, there are known a number of standard storage systems such as D8 system, DDS1 system, and DDS2 system, which utilize, as a standard magnetic recording tape, a magnetic recording tape in which a magnetic recording layer is directly formed on a support, that is known as a mono-layer type tape. In these systems, specifically selected equalizers are installed for compensating deterioration of frequency characteristics of the regenerated signals in the high frequency range as well as in the low frequency range. In the digital data storage system, the so called intersymbol interference sometimes occurs if a frequency characteristic of the regenerated signal deteriorates (in the process of recording and regeneration in the high and low frequency range), and gives an error in the regenerated signals. In order to compensate the troubles caused by the inter-symbol interference, the conventional computer data storage systems utilize the equalizer. Therefore, a magnetic recording tape for the conventional computer data storage systems should be designed to have appropriate characteristics in harmony with the characteristics of the equalizer installed in the conventional system.
The mono-layer type magnetic recording tape, that is, a magnetic recording tape in which a magnetic recording layer is directly formed on the support, has drawbacks in that it is difficult to give a high output and the running endurance is not enough.
U.S. Pat. No. 5,419,961 discloses a magnetic recording tape having a decreased thickness. For instance, it states a magnetic recording tape having a total thickness of 10 .mu.m which has a back-coating layer of 0.5 .mu.m, as well as a magnetic recording tape having a total thickness of 9.5 .mu.m which has a back-coating layer of 0.5 .mu.m. For obtaining a magnetic recording tape having improved anti-static property and running performance, the former magnetic recording tape has a back-coating layer containing a carbon black of a relatively small particle size, and the latter magnetic recording tape has a back-coating layer containing a carbon black of a relatively large particle size as well as a carbon black of a relatively small particle size. It describes further that a preferred support web may comprise a polyester, especially polyethylenenaphthalate (PEN).
From the viewpoint of recent development of the back-up tape, namely, a magnetic recording tape for computer data storage, however, the magnetic recording tape disclosed in the above-mentioned United States Patent is not enough in its performance. Mere decrease of the tape thickness cannot produce a back-up tape having satisfactory electromagnetic and running characteristics.
The present invention has an object to provide a magnetic recording tape which is appropriately employable in the conventional computer data storage systems and shows satisfactory electromagnetic characteristics and running endurance.